1. E6998-02: Internet Routing Lecture 5 Fundamentals of Routing Protocols
John Ioannidis
AT&T Labs – Research
Copyright © 2002 by John Ioannidis. All Rights Reserved.

2. Lecture 05 of E6998-02: Internet Routing
Announcements
Lectures 1-5 are available.
Guest Lecturer on 9/19: Noel Chiappa.
BE THERE!
In a different room (check the bboard right before class).
Lecture on 9/26 will be at 4:10pm in 1024.
W4180 on 9/26 will be at 2:40pm in 1127.
Homeworks submitted incorrectly. Don’t do it again.
Homework 2 will be out later tonight.
I assume that only people who submitted homeworks are in the class.
September 17th, 2002
Lecture 05 of E : Internet Routing

3. Lecture 05 of E6998-02: Internet Routing
Internet is a collection of connected nodes.
End nodes (“end systems”): hosts.
Connecting nodes (“intermediate systems”): routers.
Internet is organized as a collection of networks.
Customer networks.
ISPs.
Of varying sizes and purposes.
Within a customer network or ISP:
Cohesive routing policy.
Performance and optimal internal routing is top priority.
Between networks or ISPs:
Different routing policies.
Connectivity and abiding to policies is top priority.
September 17th, 2002
Lecture 05 of E : Internet Routing

4. Lecture 05 of E6998-02: Internet Routing
Routing Protocols
Manual configuration (static routes).
Too hard.
Does not scale.
Does not respond quickly to changing topologies.
Source routing.
Source decides path of packets (LSRR/SSRR options).
Has been considered in the past (SDRP).
Still question about how routes are computed.
Dynamic routing.
Distance-vector protocols.
Link-state protocols.
Other.
September 17th, 2002
Lecture 05 of E : Internet Routing

5. Lecture 05 of E6998-02: Internet Routing
Distance-Vector
September 17th, 2002
Lecture 05 of E : Internet Routing

6. Lecture 05 of E6998-02: Internet Routing
Distance-Vector
Variations of Bellman-Ford algorithm.
Each router starts by knowing:
Prefixes of its attached networks (“zero” distance).
Its next hop routers (how to find them?)
Each router advertises only to its neighbors:
All prefixes it knows about.
Its distance from them.
Each router learns:
All prefixes its neighbors know about.
Their distance from them.
Each router figures out, for each destination prefix:
The “distance” (how far away it is).
The “vector” (the next hop router).
September 17th, 2002
Lecture 05 of E : Internet Routing

7. Lecture 05 of E6998-02: Internet Routing
Link-State
Based on Dijkstra’s Shortest-Path-First algorithm.
Each router starts by knowing:
Prefixes of its attached networks.
Links to its neighbors.
Each router advertises to the entire network (flooding):
Prefixes of its directly connected networks.
Active links to its neighbors.
Each router learns:
A complete topology of the network (routers, links).
Each router computes shortest path to each destination.
In a stable situation, all routers have the same graph, and compute the same paths.
September 17th, 2002
Lecture 05 of E : Internet Routing

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Lecture 05 of E : Internet Routing

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